Luminance

Luminance is a photometric quantity that describes the brightness of a surface as perceived by the human eye. It is defined as the luminous intensity per unit area of the surface in a given direction, and it determines how bright a surface appears under specified lighting. In practice, luminance depends on the spectral power distribution of the light, the surface reflectance or emission, and the observer's viewing geometry.

In the CIE photometric system, luminance is derived from radiance by weighting the spectral power distribution with the standard human eye sensitivity function V(λ). The result, measured in candela per square meter (cd/m^2), is a measure of perceived brightness and is also referred to as luminance. Units are sometimes called nits.

Luminance is distinct from radiometric quantities, as it encodes human visual response. It is used in displays, lighting design, photography, and vision science. Luminance can be measured with specialized instruments called luminance meters, or inferred from camera images with proper calibration. Luminance affects perceptual contrast; the luminance contrast between two areas is (L1−L2)/L2.

Typical ranges: office displays 100–300 cd/m^2; modern HDR displays may exceed 1000 cd/m^2; surfaces viewed outdoors can reach tens of thousands of cd/m^2, and the sun is on the order of 10^9 cd/m^2 in luminance.